Due to the voltage fluctuation resulting from variable wind energy and reactive load change in an isolated wind-diesel hybrid system, an additional adaptive voltage compensation controller for static var compensator (SVC) by using double sliding-mode (SM) optimal strategy is proposed to maintain the system bus voltage stability. Considering the uncertainties of SVC model parameters and unobservable state variables, the SM algorithm is used to design the robust state observer and SVC adaptive voltage controller. The mathematical state model of isolated wind-diesel hybrid power system together with SVC is established, and then, the SM robust observer is constructed to estimate the SVC state variables. In order to improve the system bus voltage stability, an additional robust adaptive voltage controller is designed for SVC by selecting appropriate switching surface and adaptive reaching condition. The proposed control strategy is shown to be effective through opal-RT OP5600 simulator together with TIF28335 DSP for the isolated wind-diesel system under different operation point.
After decades of operations, the first batches of offshore wind farms (OWF) are approaching the end of service lifetimes, while the demand for energy transformation is continuously pushing countries around the globe to construct larger OWFs. Therefore, the old OWF's repowering and scale-expansion becomes a possibly-feasible plan which deserves more researches. This paper firstly analyzes the potential benefits of combining the repowering and expansion parts in collector system topology design, which is followed by further analysis and mathematical verifications of economy and reliability advantages for applying cross-substation incorporation (CSI) to the collector system. To achieve the optimal topology design for OWF repowering and expansion, this paper also proposes a multi-layer optimization framework that consists of an offshore substation (OS) refinement layer, an OWF partition layer, and an intra-zone cable connection layer. The whole optimization concerns the wind turbine (WT) capacity deviation between the repowered part and the expanded part, and the final output is generated with the aim of achieving optimal economy and reliability. The proposed topology is tested and compared with traditional radial and ring topologies on a benchmark collector system, and the results verify the high performance in the economy (both short-term and long-term) and reliability.
Wind power may become the key development area of the renewable energy industry planning. Due to the fluctuated wind output power, the system parameter uncertainties and load disturbances, these may lead to the unstable operation of renewable energy power system for the larger voltage deviation. Therefore, the novel control strategy is proposed for the load side converter (LSC) of double-fed induction generator (DFIG) to improve the stability of power system, which is designed by taking advantage of adaptive sliding mode (SM) method and sliding mode observer (SMO). Furthermore, the static synchronous compensator (STATCOM) is also controlled through the designed adaptive SM reactive power controller, which can supply the reactive power compensation for the interconnected wind-diesel power system. Moreover, the LSC state variables need to be estimated by the SMO, which is constructed according to the established mathematical state model. Based on the estimated state variables, the designed reactive power controller is established for LSC by adaptive SM algorithm and fuzzy algorithm. The reactive power controller includes two parts. One is the designed SM controller by taking advantage of the estimated state variables and state control model. The other is micro-regulated part and the control signal is calculated by using fuzzy algorithm according to the system voltage deviation online. Experiment studies are performed by using RTDS to verify the effectiveness of the proposed strategy and assure the stability of wind-diesel power system under different operation conditions.
Due to frequency and voltage fluctuation resulting from the renewable energy variety, parameter uncertainty, and system disturbance, a novel coordinated control strategy is proposed to improve the dynamic stability of isolated wind-diesel hybrid power system. So, the frequency and voltage can also be regulated by taking advantage of adaptive sliding mode (SM) method and disturbance observer (DOB), which are based on the established whole hybrid system mathematical model. First, the DOB is constructed to estimate the source-load disturbance for isolated wind-diesel power system model. Second, the coordinated control strategy is constructed according to the estimated source-load disturbance value and state control model by using adaptive SM algorithm. Then, the output vector signal of adaptive SM controller is added to the synchronous generator side and the static synchronous compensator, respectively; therefore, the proposed coordinated control strategy can regulate the frequency and voltage simultaneously. Experiment studies are performed by real-time digital simulator to validate the effectiveness of the proposed coordinated control strategy for the isolated hybrid system under different operation points.
Availability of wind turbine (WT) is an essential parameter both for wind project feasibility analysis and operation and maintenance optimizations, but in offshore wind farms, it is greatly affected by accessibility which has been seldom considered in traditional reliability evaluations. This paper presents a Markov-chain-based availability assessing model of offshore WT considering accessibility problems. Based on the procedure analysis of corrective maintenance offshore, the contribution of poor accessibility to availability is summarized into two independent aspects: stochastic offshore weather and inadequate maintenance resources. A three-state model of offshore WT is established. A Poisson-process-based algorithm is presented to calculate a transition rate of the three-state model. A three-integer field representing a wind farm with NWT WTs is defined for the Markov chain. Mean availability of WT is obtained by solving a Markov transition matrix. The results of case study show that the proposed model provides an effective approach for the availability assessment of WT in a multi-turbine wind farm influenced by stochastic offshore weather and different logistic policies.
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